The clock is ticking for you to use one key strategy for your individual retirement accounts before it disappears for good.

If you converted money in a traditional IRA to a post-tax Roth IRA in 2017, you have until Oct. 15 to undo that transaction.

But this will be the last year that you can change your mind. That is because the Tax Cuts and Jobs Act that was passed last year eliminated the ability to reverse an IRA conversion in 2018 and beyond.

"At a minimum, you should evaluate it to see if you want to keep it," said Ed Slott, founder of Ed Slott & Co. and an expert on IRAs.

Moving funds from your traditional pre-tax IRA to a post-tax Roth IRA generally leaves you with a tax bill for the money that is converted. Then, when you eventually withdraw those funds from your Roth IRA in retirement, you will not have to pay tax on that money.

Because the market has mostly been up, your retirement investments most likely have also seen a boost.

"If you have that 2017 conversion in the market and you made a lot of money, leave it alone," Slott said. "That's all tax-free gains."

When to act

But you may have lost money or stayed flat. At that point, you may want to reverse your decision.

That way, you can undo it at 2017 tax rates, which are higher, Slott said. You then have the opportunity to reconvert those funds at 2018 tax rates, which might be lower, he said.

"If you want to reconvert those same funds, you have to wait at least 30 days," Slott said.

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You can decide to reverse part of the transaction based on how much you will pay in taxes in 2017, Slott said.

Remember that once you undo a Roth conversion, you are bringing those funds into a taxable account.

"If you had big gains in 2017, don't reverse them unless you don't have the money to pay the tax," Slott said.

Don't wait

If you plan to reverse a conversion you previously did in 2017, you should do that as soon as possible, Slott said, rather than wait closer to the Oct. 15 deadline.

"I have a feeling the financial institutions will be getting barraged with requests at the last minute, kind of like the post office on Tax Day," Slott said.

And if you miss that Oct. 15 deadline, you do not get another chance.

"It's not the worst thing in the world. You still have a Roth," Slott said. "But if you want to give it one more look to see if the conversion worked out, this is your chance and the window is closing."

One important thing to note is that this only applies to transactions that were done in 2017. Roth conversions that were done in 2018 are permanent and cannot be reversed.

If you already filed your taxes for 2017 in April, you can still undo your conversion. But you will have to file an amended tax return with the IRS. The last day to file your 2017 taxes is also Oct. 15.

"See if you've lost money," Slott said. "If you have, I would undo it.

"This is your last chance to avoid paying taxes on the value that no longer exists — and last chance ever."

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